1. Field of the Invention
This invention relates to an intake system for an internal combustion engine, and more particularly to an intake system for an internal combustion engine in which the engine output power is improved by the kinetic effect of intake air.
2. Description of the Prior Art
As is well known, a negative pressure wave generated in an intake system of an internal combustion engine upon the initiation of each intake stroke is propagated upstream of the intake system and is then reflected at an end of the system opening to the atmosphere or to a surge tank disposed on an upstream side of the intake system toward the intake port as a positive pressure wave. By arranging the intake system so that the positive pressure wave reaches the intake port immediately before closure of the intake valve to force intake air into the combustion chamber, the volumetric efficiency can be improved. There have been known various intake systems in which such inertia effect or resonance effect of intake air is used for improving the volumetric efficiency. However, the period of oscillation of the pressure wave in the intake passage can be matched with the period of opening and closure of the intake valve to obtain a sufficient inertia effect of the intake air only within a limited engine speed range which depends upon the shape of the intake passage. There has been proposed an intake system in which, for instance, the length of the intake passage is changed according to the engine speed in order to obtain an inertia effect of intake air over a wider engine speed range. For example, in the intake system disclosed in Japanese Unexamined patent publication No. 56(1981)-115819, each of the discrete intake passage portions leading to the respective combustion chambers is bifurcated to form a long passage portion and a short passage portion both opening to a surge tank or the like at the upstream end, and an on-off valve is provided in the short passage portion to open the short passage portion in the high engine speed range to shorten the effective length of the discrete intake passage portion, thereby obtaining a sufficient inertia effect of intake air in the high engine speed range in addition to a low engine speed range. (See FIG. 6 of the Japanese unexamined patent publication described above.)
In the intake system described above, the volumetric efficiency for one cylinder is improved by the inertia effect of intake air generated by pressure propagation only in the discrete intake passage portion leading to the cylinder. If the pressure propagation in the discrete intake passages leading to other cylinders can be effectively utilized, the volumetric efficiency will be further improved.
Thus, we have proposed in Japanese Unexamined patent publication No. 59(1984)-275487 an intake system for a multicylinder internal combustion engine in which the inertia effect of intake air can be effectively utilized to improve the volumetric efficiency in both the low engine speed range and the high engine speed range, and at the same time, the inertia effect of intake air in each discrete intake passage portion can be enhanced by the pressure wave in at least one of the other discrete intake passage portions especially in high engine speed ranges. The intake system has an intake passage comprising a common passage portion opening to the atmosphere, a surge tank connected to the downstream end of the common passage portion and a plurality of discrete passage portions branching from the surge tank and respectively connected to the cylinders. At least one interconnecting passage is provided to communicate each of the discrete passage portions with at least one of the other discrete passage portions at a portion downstream of the surge tank, and an on-off valve is disposed at each junction of the interconnecting passage with the discrete passage portions to open and close each junction. The on-off valve is opened at least when the engine speed exceeds a predetermined value.
In order to further improve the volumetric efficiency in the intake system of the type described above, it is preferred that the effective cross-sectional area of the intake passage be narrowed to increase flow speed of intake air in the low engine speed range in which the amount of intake air is relatively small. Generally, the volumetric efficiency is affected also by flow speed of intake air and intake resistance, and it has been known that the best kinetic effect of intake air can be enjoyed when the flow speed of intake is approximately 60 m/sec. On the other hand, if the effective cross-sectional area of the intake passage is excessively small in the high engine speed range in which the amount of intake air is relatively large, the intake resistance is increased to lower the volumetric efficiency. Our investigation on the relation between the flow speed of intake air and the intake resistance has revealed that the flow speed of intake air at engine speeds near the engine speed corresponding to the maximum horsepower is preferably not higher than 100 m/sec.